In the text hereafter, what is referred to as chemical looping oxidation-reduction combustion (CLC) is an oxidation-reduction or redox looping process on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced. The reactors operate under fluidized bed conditions and the active mass circulates between the oxidation reactor and the reduction reactor. The circulating fluidized bed technology is used to enable continuous change of the active mass from the oxidized state thereof in the oxidation reactor to the reduced state thereof in the reduction reactor.
In a context of increasing world energy demand, capture of carbon dioxide (CO2) for sequestration thereof has become an imperative necessity in order to limit greenhouse gas emissions harmful to the environment. The CLC process allows to produce energy from hydrocarbon-containing fuels while facilitating capture of the CO2 emitted during the combustion.
The CLC process consists in using redox reactions of an active mass, typically a metal oxide, for splitting the combustion reaction into two successive reactions. A first oxidation reaction of the active mass, with air or an oxidizing gas, allows the active mass to be oxidized.
A second reduction reaction of the active mass thus oxidized, using a reducing gas, then allows to obtain a reusable active mass and a gas mixture essentially comprising CO2 and water, or even syngas containing hydrogen and carbon monoxide. This technique thus enables to isolate the CO2 or the syngas in a gas mixture practically free of oxygen and nitrogen.
Chemical looping combustion enables to produce energy from this process, in form of vapour or electricity for example. The feed combustion heat is similar to the heat encountered in conventional combustion. It corresponds to the sum of the reduction and oxidation heats in the chemical loop. The heat is generally extracted through exchangers arranged within or on the wall of, or inserted in the combustion and/or oxidation enclosures, on the fumes lines, or on the metal oxide transfer lines.
In addition to the advantage of recovering the combustion heat to produce energy, it is important to be able to control the temperature in a CLC process. Indeed, the temperature of the reactions in the oxidation and reduction zones needs to be controlled for safety reasons and in order to guarantee good performances of the process. This heat recovery for controlling the process heat is preferably achieved by heat exchange with the active mass circulating in the loop.
This heat recovery can be achieved at the walls of the redox reactors in a CLC process. However, this configuration may not be optimal, notably because the thermal exchanges are limited due to an exchange surface limited to the walls, and because only part of the particles is in contact with this exchange surface, but also because it is not always possible to modify the flow rate of the oxygen-carrying solid in these reactions zones for the sole purpose of heat exchange control. Indeed, the oxygen-carrying solid flow is directly related to the amount of oxygen used for combustion, and any change in the flow rate of the oxygen-carrying solid has an impact on the progress of the reactions, which may for example cause fuel management and/or reaction stoichiometry problems, and eventually affect the process yield.
Using heat exchangers outside the combustion and/or oxidation enclosures may be considered.
In general terms, such fluidized bed heat exchange devices are known in some fields such as circulating fluidized bed thermal power plants, and they are identified as fluidized bed heat exchangers (FBHE) or external heat exchangers (EHE). They generally come in form of a vessel comprising an inlet for a solid in form of particles, a fluidization device using a gas, a gas outlet and a solid outlet (Q. Wang et al., Chemical Engineering and Processing 42 (2003), 327-335).
To achieve heat exchange, tube bundles are conventionally provided in the fluidized bed so as to generate an exchange surface between the fluidized bed and a heat-carrying fluid circulating in the tubes. Conventionally, in the field of energy, in particular in thermal power plants, the heat-carrying fluid is pressurized boiler water that is either heated or at least partly vaporized, or overheated in the tube bundles of the heat exchanger. These exchangers generally operate with an overflow mechanism for discharge of the solid, as illustrated for example in patent U.S. Pat. No. 4,716,856 or by Wang et al. (Chemical Engineering and Processing 42, 2003, pp. 327-335). Using an overflow pipe implies that the volume of the fluidized bed is constant and therefore that the exchange surface with the fluidized bed is constant. This makes it impossible to modulate the heat recovery through a variation of this surface and requires modulating the operating parameters such as, for example, the flow of solid passing through the exchange zone.
A solid flow control device such as a mechanical valve, as described in patent EP-0,090,641 A2, can then be used. One drawback of this option is the use of a mechanical device for controlling the flow of solid. This type of device is particularly limitative in processes operating at high temperatures, such as a CLC process, and it can lead to reliability problems inherent in the implementation of a mechanical device comprising mobile parts in a bed of fluidized particles abrasive at high temperatures.
In order to regulate the flow of solids, it is possible to use nonmechanical valves such as the pneumatic valves described in patent application WO-2011/007,055, used to control the circulation of the solid active mass particles in a CLC process. This type of pneumatic valve allows to tackle the problem of temperature and abrasion. However, the smooth operation of these pneumatic valves is limited to the use of certain particle classes, namely the particles of group B in Geldart's classification.
FIG. 1 (extracted from K. Shakourzadeh, Techniques de fluidisation, réf. J3390, Techniques de l'Ingénieur, p. 10) illustrates a particular system allowing to modify the flow of solid passing through an external fluidized bed heat exchanger operating with an overflow pipe, optionally with a valve on the solid stream entering the exchanger. This system is installed in a conventional circulating bed coal (air) combustion unit comprising a combustion reactor 10 from which a gas mixture containing the combustion gases and solid particles is sent to a cyclone 20. An external heat exchanger 50 equipped with an overflow pipe is arranged between cyclone 20 and reactor 10. According to this system, only part of the solid stream recovered in the bottom of cyclone 20 is sent through a pipe 40 to heat exchanger 50 prior to returning to reactor 10, the other part of the solid stream being sent back through the agency of a siphon/return leg assembly 30 to reactor 10. A valve is generally provided on pipe 40. Heat exchange is thus controlled by modifying the solid flow passing through exchanger 50. This configuration, consisting in a solid flow by-pass, complexifies the architecture of the unit and the process in which it is implemented, all the more so as it involves using a valve on the circuit of the solid so as to orient it towards the by-pass.
Generally, solutions consisting in modulating the flow of solid by means of a valve in order to modify the heat flux can affect the proper operation of the process and/or limit the operation thereof. For example, if it is desired to limit or even to cancel the exchange between the solid and the heat-carrying fluid, either the flow of solid needs to be limited or stopped, which can lead to the slowdown or standstill of the unit, or the circulation of the heat carrier has to be limited or stopped, which may damage the tube bundles of the heat exchanger.
There is thus a need for an improved CLC process wherein heat exchanges with the circulating oxygen carrier can occur within an external exchanger without using a valve on the solid flow for modifying the amount of exchanged heat. This need is all the more markedly felt as the temperature of the circulating fluidized bed of a CLC process can be substantially higher than with a conventional circulating fluidized bed (CFB) combustion process, depending on the nature of the oxygen carrier and on the feed treated, which makes it difficult to install a mechanical valve.